Influence of carbon doping in the vortex matter properties of MgB${}_2$

Abstract

We have studied the vortex-matter phase diagram of low C doped MgB${}_2$ superconductor. The significant finding is the appearance of a peak effect, which, as both our global magnetization and local ac susceptibility measurements revealed, is placed well below the $H_{c2}$-line. The absence of significant bulk pinning below the onset line $H_{on}(T)$ of the peak effect implies that the Bragg glass phase is present for $H<H_{on}(T)$. Interestingly, the unexpected absence of bulk pinning above the end point line $H_{ep}(T)$ of the peak effect implies the presence of a slightly pinned vortex phase in the regime $H_{ep}(T)<H<H_{c2}(T)$. In addition, the observed increase of the $H_{c2}^c(0)$ since the carbon doped MgB${}_2$ becomes more dirty and the reduced anisotropy, in comparison to the pure MgB${}_2$, makes the C-doped MgB${}_2$ more favorable for practical applications.